Dragline excavators are commonly employed in tasks such as the removal of overburden in preparation for open cut coal mining. In these, a bucket capable of scooping typically 50 cubic meters of soil is actioned off a boom, which might be typically 50 meters in length. The draglines are sold rated at a particular suspended load for operating purposes. The suspended load is the total of the weight of the load of overburden picked up, the weight of the bucket, and, additionally, the weight of the rigging which supports the bucket off the boom and couples to it to enable its action, the rigging being the assembly of elements and cable by which the bucket is suspended and operated.
The design of dragline rigging is well settled. It has remained in common use, largely unchanged, over a substantial number of years. The assembly supporting the bucket off its lifting cable, and interlinked with the drag line bucket, is typically an array of linkages between a hoist socket and a trunnion link, respective elements being interconnected by swivel pins of the like. The bearing surfaces between these elements wear over time, and they require regular servicing. Normally the dragline excavator is operated continuously, being shut down only every two to three weeks for a matter of hours, typically eight, in which to effect servicing. The down time represents a significant operational cost and management seeks to minimise it. Servicing the wear points is pre-planned. An inspection of expected wear points is made during a short stoppage, some days before a service is expected. This enables acquisition of what replacement parts might be needed in preparation for their being installed.
Either or both of a reduction in weight of the rigging, or the down time taken in servicing it, will have a significant impact on the overall economics of drag line operation.